Whole tooth crowns of rats, labelled with 32P, were incubated in buffer solutions in the presence of caries-inducing K-1 streptococci of human origin. The changes associated with sucrose fermentation were: decreased extra- cellular ρH, increased phosphorus release from the teeth, increased phophorus content and a markedly increased 32P label of the organisms. An addition of Na2HPO4 to the fermenting system, whilst increasing the cellular total phosphorus content further, reduced the cellular P label. When used in high quantity, the Na2HPO4 also prevented pH drop and lessened the 32P release from the teeth. The transfer of 32P from the teeth into the cariogenic bacteria was reduced by addition of Na2HPO4